I work in school in the mornings and do our finance admin in the afternoons (purchasing, payments).
Coach trips alone have skyrocketed post-COVID. A trip that was previously £300 is now £500 or more. That's for relatively short trips, not residentials or anything.
As for the budget for other things (stationary, sports kit) well it's down to your school as to whether or not they fit in their budget. Stationary costs went up 40% last year, and have only just come down a bit for some items.
The LA have just tripled our school dinner charges. We are fighting it whilst simultaneously desperately looking for another company, but we will need to pay their extortionate rates for at least 6 months because we simply can't change that quickly.
The government is still requiring that schools do X number of hours of post-covid catchup but they no longer fund them (they fully funded the first year, half funded the second year, and now they only pay 25%. For something THEY are requiring we do, this is insane)
I know our school is already in a budget deficit for NEXT school year.
Honestly, if spending the money to send your children on trips is a hardship, don't send them. Give feedback to the school as to why, but they aren't charging you because they want to make your life difficult. They're doing it because they're on the brink of financial disaster.